An axial flow gas turbine engine generally comprises, in axial flow series, an air intake, a propulsive fan, an intermediate pressure compressor, a high pressure compressor, combustion equipment, a high pressure turbine, an intermediate pressure turbine, a low pressure turbine and an exhaust nozzle.
The turbines typically comprise a set of axially alternating stationary nozzle guide vanes and rotatable turbine blades. The nozzle guide vanes and turbine blades are mounted generally in a ring formation, with the vanes and the turbine blades extending radially outwardly. Gases expanded by the combustion process in the combustion equipment force their way into discharge nozzles where they are accelerated and forced onto the nozzle guide vanes, which impart a “spin” or “whirl” in the direction of rotation of the turbine blades. The gases impact the turbine blades, causing rotation of the turbine.
The torque or turning power applied to the turbine is governed by the rate of gas flow and the energy change of the gas between the inlet and outlet of the turbine blades.
A gap exists between the blade tips and casing, which varies in size due to the different rates of expansion and contraction of the blade and casing. To reduce the loss of efficiency through gas leakage across the blade tips, a shroud is often fitted. This consists of a small segment at the tip of each blade which together form a peripheral ring.
However, even with a fitted shroud, tip leakage reduces efficiency in a number of ways. Work is lost when the higher pressure gas escape through the tip clearance without being operated on in the intended manner by the blade (for compressors the leakage flow is not adequately compressed and for the turbines the leakage is not adequately expanded). Secondly, the leakage flow from the pressure side produces interference with the suction side flow. The difference in the orientation and velocity of the two flows results in a mixing loss as the two flows merge and eventually become uniform. Both types of losses contribute to reduction in efficiency.
The problem of tip leakage has been investigated for many years and no effective and practical solution has been found other than reducing the tip clearances. Most current solutions involve active changing of the tip clearance by adjusting the diameter of the engine case liner.
It has now been found through computational fluid dynamics (CPD) that the overtip leakage flow from the high pressure turbine also has an adverse effect of the intermediate pressure turbine vane inlet conditions and thereby reduces efficiency.
It is an object of the present invention to seek to improve the efficiency of a turbine.
It is a further object of the present invention to seek to address the adverse effects of over tip leakage on a turbine.